1. Field of the Invention
The invention relates to polymer waveguides in which a core layer and a cladding layer each formed of a polymer material and the interface between the core layer and the cladding layer can be evenly formed with good adhesion between the core layer and the cladding layer and a cladding layer having even thickness can be formed on the core, and a process for producing the same.
2. Prior Art
Polymer waveguides have features including that they can be produced in a simple manner, the area can be easily increased, and the cost could be lowered. By virtue of these features, polymer waveguides are expected to be put to practical use.
For example, acrylic, polystyrene, epoxy, polyimide, silicone, and polysilane materials have been studied as materials for polymer waveguides. These materials are desired to be less likely to undergo a change in characteristics, for example, refractive index or coefficient of thermal expansion, upon a change in temperature.
In view of the above requirements, among the above polymer materials, polyimide, epoxy, and polysilane materials have drawn attention, and an improvement in these polymer materials has been attempted.
Examples of attempts include, for example, the use of a linear (straight-chain) polysilane material for optical applications as shown in FIG. 1 (see Japanese Patent Laid-Open No. 222234/1994), the use of amorphous polysilane (see Japanese Patent Laid-Open No. 287916/1999), and the use of linear polysilane or branched polysilane (see Japanese Patent Laid-Open No. 262728/1996).
FIG. 1 is a perspective view of the appearance of a polymer waveguide using a linear polysilane material.
In FIG. 1, numeral 50 designates a core of methylphenylpolysilane, and numeral 51 a cladding of polysiloxane.
The above prior art techniques, however, involve the following problems.
(1) The refractive index significantly changes upon a change in ambient temperature. This in turn leads to a significant change in optical characteristics (such as propagation mode, power distribution, and wavelength characteristics) of optical circuits comprising polymer waveguides using the above polymer material and thus makes it impossible to provide desired properties.
(2) In soldering an electronic component or an optical component to a portion around the polymer waveguide at a temperature around 200° C., the refractive index of the polymer waveguide is disadvantageously changed from the initial value to a value different from the initial value, and, even when the temperature is returned to ambient temperature, the refractive index of the polymer waveguide cannot be returned to the initial value. This leads to a change in optical characteristics of the optical circuit comprising the polymer waveguide as described in the above item (1).
(3) A method for forming a three-dimensional waveguide by applying ultraviolet light to a film using a linear polysilane material has been proposed. This method, however, involves the above problems (1) and (2) and, in addition, for example, a problem of the dependency of refractive index upon polarized light, and thus has not been put to practical use yet. Further, even when an attempt is made to cause a significant change in refractive index of the polymer film through the application of ultraviolet light with quick response, a change in refractive index is discontinuous in relation to irradiation energy. In addition, in order to achieve a significant change in refractive index, the level of irradiation energy should be enhanced.
(4) A polymer waveguide is generally prepared as follows. A solution of a polymer material, comprising a polysilane compound for a core with a high refractive index, in an organic solvent is spin coated onto a buffer layer with a low refractive index. The coating is then heat treated at a solder reflow temperature (a temperature of 250 to 300° C.) to cure the coating and thus to form a polymer layer with a high refractive index. A solution of a polymer material, for a cladding with a low refractive index, in an organic solvent is coated onto the polymer layer, and the coating is then heat treated at a solder reflow temperature. In this case, when the heat treatment of the coating for the cladding is started, disadvantageously, the polymer material solution for the cladding is repelled by the surface of the polymer layer for the core resulting in uneven surface of the cladding, or otherwise the transparency of the underlying polymer layer for the core is lost. This increases light scattering loss.
(5) The cladding layer formed by the method described in the above item (4) is likely to be separated upon the heat treatment, that is, has poor adhesion.